1. Electrical Stopping Power Introduction to Resistance

2. Resistance Resistance: is a measurement of how strongly
an object will oppose current
An object’s resistance depends on several factors:
Resistivity
Length
Some materials are better conductors than others.
Longer objects are more difficult to pass current through.
Cross-sectional Area
Temperature
Wider objects allow current to pass more easily.
If the object is warm, the
molecules inside will be bouncing
around more – opposing current.

3. Equation
Unit for Resistance
is the: OHM Ω
R = resistance

4. Resistance in Wires The “IDEAL” conductor has a LOW RESISTIVITY and is
We would like to have wires (conductors) that allow
CURRENT to flow from one place to
another EASILY
“IDEAL” conductor  minimum RESISTANCE
The “IDEAL” conductor has a
LOW RESISTIVITY and is
SHORT, WIDE, and COLD
WIDE or NARROW?
LONG or SHORT?
HOT or COLD?
COLDER conductors have lower resistance because:
atoms are not as energetic – don’t interfere with electrons
WIDER conductors have lower resistance because:
electrons have more room to pass!
SHORTER conductors have lower resistance because:
electrons do not have to travel as far!

5. Plug in the factor each changes by
Example #1
If the length of the wire is doubled, what happens to the resistance of the wire?
Plug in the factor each changes by
Doubles!!!

6. Plug in the factor each changes by
Example #2
What happens to the resistance of a wire if the resistivity is cut in half, and the cross sectional area is increased by a factor of 8?
Plug in the factor each changes by
¼ As Large

7. Plug in the factor each changes by
Example #3
If the length doubles, the resistivity is decreased to ¼ its original value, and the cross sectional area is tripled, what happens to the resistance of the wire?
Plug in the factor each changes by
1/6 As Large

8. If the length of the wire is… what happens to the resistance?
Doubled
Quadrupled
If the resistivity of the wire is changed by a factor of… what happens to the resistance?
3times greater
½ as large
If the surface area of the wire is… what happens to the resistance?
10times larger
¼ as large